This application claims priority under 35 U.S.C. xc2xa7119 to Great Britain Patent Application No. 0021749.7 filed Sep. 2, 2000, and Great Britain Patent Application No. 0114493.0 filed Jun. 14, 2001.
Not applicable.
Not Applicable.
1. Field of the Invention
The present invention relates to a chain drive of the kind comprising a chain that meshes with a sprocket, and particularly but not exclusively to a chain drive designed to reduce undesirable noise generated during meshing.
2. Description of the Related Art
A chain drive typically comprises at least two toothed sprockets and a chain comprising links that are pivotally connected to one another by pivot pins. The chain engages the teeth on the periphery of each sprocket and rotation of one sprocket is transmitted to the other via the chain. When the chain meshes with the sprockets, undesirable noise is generated as a result of the collision of the chain with the sprocket and the resultant vibration of the chain and sprocket.
The collision between the chain and sprocket happens rapidly during meshing as a result of what is known as xe2x80x9cchordal actionxe2x80x9d or the xe2x80x9cchordal effectxe2x80x9d. These terms are used to describe the uncontrolled and rapid movement of the chain links relative to the sprocket teeth during meshing and unmeshing. The movement arises from the fact that the pitch line of the chain comprises a plurality of straight lines or chords that combine to form part of a polygon rather than a circle (as would be the case for a flexible belt). The rapid and uncontrolled movement of the chain links results in them impacting on the sprocket with a high velocity thereby generating noise.
Many attempts have been made to reduce the noise in a chain drive. For example, U.S. Pat. No. 5,127,884 describes a chain drive comprising a chain having hinged joints each equipped with a torsional spring and a sprocket with cam manipulated teeth. The hinged joints and torsional springs are designed to resist hinging of the links and therefore prevent the links falling freely on to the sprocket. This reduces the impact velocity and associated noise attributable to the chordal effect. In order to reduce the noise further, support pads are also provided on the sprocket to prevent sliding of the chain links. The provision of the torsional spring at each joint significantly increases the cost of the chain.
U.S. Pat. No. 4,261,214 discloses a sprocket equipped with a cushion ring having a larger diameter than the nominal sprocket diameter. The cushion ring cushions the fall of the chain links on to the sprocket in order to reduce the noise generated by the chain. However, such rings have a tendency to wear during use and require regular replacement. Similarly, U.S. Pat. No. 5,360,378 describes the use of rubber rings on the face of the sprocket, although the chain links do not come into contact with the rings. Noise reduction is stated to be achieved by the absorption of sprocket vibration by the rubber rings.
U.S. Pat. No. 4,650,445 describes a chain having links with longitudinally spaced apertures that are offset with respect to one another to vary the timing between the links falling onto the sprocket. The links pivot about a pair of pins that are located in the apertures and have adjacent rocking surfaces to form the pivot point. The chordal action is varied between links so that the resulting noise pattern is irregular and hence is less objectionable to the human ear.
DE-A-19543287 describes a chain drive that seeks to avoid xe2x80x9cflank shockxe2x80x9d that occurs as a result of the chordal action. The sprocket wheel is designed to allow tangential guidance of the chain as it meshes. This is provided for by resilient support rings or spring-loaded support rams on both sides of the sprocket teeth. These additional components provide for added complexity and increase the manufacturing costs. In the case of the spring-loaded support rams the point of contact of a chain link with the ram during meshing moves so that its distance from the pivot axis of the pin increases. At the same time a progressively increasing torque is applied by the ram to the chain plate.
Although there are many attempts in the prior art to reduce the noise generated by chordal action, none of these has been entirely successful. It is an object of the present invention to provide an improved chain drive which reduces unwanted noise generated by the chordal action.
According to a first aspect of the present invention there is provided a chain drive comprising a chain having a plurality of chain links, each chain link having two apertures spaced apart along the length of the chain, each chain link being connected to adjacent links by first and second pivot pins that pass through said apertures such that the links articulate relative to one another about the pins, and a sprocket having a plurality of peripheral teeth that mesh with the chain, wherein the sprocket and at least one chain link each have a bearing surface, at least one of the bearing surfaces being arcuate such that during meshing of said chain link the bearing surfaces are brought into contact so that the instantaneous center of rotation of the meshing chain link passes from the first pivot pin of the link to the point of contact of the bearing surfaces before passing to the second pivot pin of the link and a resulting rocking movement of the chain link relative to the sprocket is thereby supported.
As the meshing link is supported at the bearing surfaces by virtue of the rolling contact between the two, its motion is controlled and the velocity of impact with the sprocket is reduced.
It has been found that with prior art chains, the chordal action can be attributed to the instantaneous center of rotation (hereinafter xe2x80x9cICRxe2x80x9d) of a chain link changing abruptly from one pivot pin axis to another during meshing with the sprocket. The movement of the chain link is uncontrolled and rapid and results in high velocity impact with the sprocket thereby generating undesirable noise. When a chain link and sprocket are provided with bearing surfaces (at least one of which is arcuate) that bear against each other during meshing such that there is a moving point of contact, the ICR moves smoothly from one pin of the link to the other via the point of contact between the bearing surfaces, thus reducing the impact velocity of the pivot pin (or more typically of a roller provided around the pivot pin) with the sprocket. This reduces the amount of noise generated.
The bearing surface of the sprocket is ideally fixed in a radial direction and/or substantially incompressible.
The arcuate bearing surface may be provided on the chain link at a position along its length between the first and second pivot pin so that during meshing there is a moving point of contact between the two bearing surfaces in the region between the two pivot pins. Alternatively the arcuate bearing surface may be provided on the sprocket to one side of the tooth such that during meshing the bearing surface on the chain link is in moving contact therewith.
Preferably, the bearing surface on the chain link is provided by a convex arcuate profile defined on a bottom flank of a link plate of the chain link.
In a preferred embodiment the teeth of the sprocket are configured to allow the longitudinal axis of one of the pins to move below the pitch diameter of the sprocket. A root between adjacent teeth of the sprocket is ideally undercut to allow the longitudinal axis of the pin to move below the pitch diameter of the sprocket.
Preferably, the teeth each have a flank with a middle section that is substantially parallel to the diameter of the sprocket and is designed to contact a roller of the chain link during meshing. The roller of the chain link slides down the middle section of a tooth flank during meshing. The bearing surface and the teeth are configured so as to ensure that the bearing surfaces are brought into contact before the roller contacts teeth, thus allowing the chain link to rock about the bearing surface contact point before a second roller of the chain link is brought into contact with the next teeth on the sprocket.